1. Field of the Invention
The present invention relates to a giant magnetoresistive (GMR) head with a keeper layer that shunts no sense current and more particularly to a keeper layer that is electrically insulated from other layers of a spin valve sensor so that sense current is not conducted through the keeper layer.
2. Description of the Related Art
The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm above the rotating disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly mounted on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent the ABS to cause the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
In recent read heads a spin valve sensor is employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer, and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to an air bearing surface (ABS) of the head and the magnetic moment of the free layer is located parallel to the ABS but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with the pinned and free layers. When the magnetization of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetization of the pinned and free layers are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to sin.sup.2.theta., where .theta. is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals. A spin valve sensor is characterized by a magnetoresistive (MR) coefficient that is substantially higher than the MR coefficient of an anisotropic magnetoresistive (AMR) sensor. For this reason a spin valve sensor is sometimes referred to as a giant magnetoresistive (GMR) sensor.
It is important that the magnetic moment of the free layer be directed substantially parallel to the ABS when the sensor is in a quiescent state. The quiescent state occurs when the sense current is conducted through the sensor without an applied field from the rotating disk. The parallel position corresponds to a zero bias point on a transfer curve of the sensor. The transfer curve of the sensor can be plotted as GMR effect (ratio of change in resistance to resistance of the sensor) as a function of applied field. Applied fields from the rotating disk move the magnetic moment of the free layer up or down from the parallel position depending upon whether the applied field is positive or negative (representing ones and zeros in a digital computing scheme). This rotation, relative to the pinned magnetic moment of the pinned layer, causes scattering of spin dependent electrons at interfaces of certain layers in the sensor which results in resistance changes of the sensor. These resistance changes cause potential differences which can be processed by the processing circuitry as read signals.
During the quiescent state there are magnetic forces acting on the free layer that urge the magnetic moment of the free layer to rotate from the parallel position to the ABS. If the magnetic moment of the free layer is not parallel to the ABS in the quiescent state read signal asymmetry will occur which means that the potentials of the positive and negative read signals are unequal. This results in a reduced read signal. Accordingly, there is an ongoing effort to balance the magnetic forces acting on the free layer in the quiescent state. These magnetic forces are a ferromagnetic coupling field H.sub.C exerted by the pinned layer on the free layer, sense current fields H.sub.SC exerted by the pinned and spacer layers on the free layer and a demagnetization field H.sub.D exerted by the pinned layer on the free layer. The ferromagnetic coupling is antiparallel to the sense current and demagnetization fields. Unfortunately, in any practical sensor scheme the combination of the sense current and demagnetization fields is greater than the ferromagnetic coupling field which results read signal asymmetry. A reduced net demagnetization field on the free layer would promote read signal symmetry.
Another problem that can occur with spin valve sensors is a loss of exchange coupling between the pinning and pinned layers when the sensor is heated by an unwanted event. The sensor can encounter elevated thermal conditions by electrostatic discharge (ESD) from an object or person, or by contacting an asperity on a magnetic disk. When this occurs the blocking temperature (temperature at which magnetic spins of the layer can be easily moved by an applied magnetic field) of the antiferromagnetic layer can be exceeded, resulting in disorientation of its magnetic spins. The magnetic moment of the pinned layer is then no longer pinned in the desired direction.
Efforts continue to increase the spin valve effect of GMR heads. An increase in the spin valve effect equates to higher bit density (bits/square inch of the rotating magnetic disk) read by the read head. Promoting read signal symmetry with regard to the free layer and maintaining thermal stability of the pinning layer are important factors.